Chapter 8

1. Chapter 8 Alkenes and Alkynes II: Addition Reactions

2. Addition Reactions of Alkenes

3. p-bond s-bond 2 s-bonds 1A. How To Understand Additions to Alkenes • This is an addition reaction: E–Nu added across the double bond. Bonds broken Bonds formed

4. Since p bonds are formed from the overlapping of p orbitals, p electron clouds are above and below the plane of the double bond. p electron clouds

5. Electrophilic • electron seeking • C=C and C≡C p bonds are particularly susceptible to electrophilic reagents (electrophiles) • Common electrophile • H+, X+ (X = Cl, Br, I), Hg2+, etc.

6. In an electrophilic addition, the p electrons seek an electrophile, breaking the p bond, forming a s bond and leaving a positive charge on the vacant p orbital on the adjacent carbon. Addition of B– to form a s bond provides an addition product.

8. Electrophilic Addition ofHydrogen Halides to Alkenes: Mechanism and Markovnikov’sRule • Mechanism:

9. Mechanism • Sometimes do not go through a “free carbocation”, may go via:

10. Markovnikov’s Rule • For symmetrical substrates, there is no regiochemistry problem.

11. Markovnikov’s Rule • But for unsymmetrical substrates, two regioisomers are possible.

12. Markovnikov’s Rule • In the electrophilic addition of an unsymmetrical electrophile across a double bond of an alkene, the more highly substituted and more stabilized carbocation is formed as the intermediate in preference to the less highly substituted and less stable one.

13. Markovnikov’s Rule • Thus: Note: carbocation stability  3o > 2o > 1o

14. Addition of Hydrogen Halides • Addition of HCl, HBr and HI across a C=C bond. • H+ is the electrophile.

15. Energy profile:

16. 2A. Theoretical Explanation ofMarkovnikov’s Rule • One way to state Markovnikov’s rule is to say that in the addition of HX to an alkene, the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms.

17. ☓ Step 1 Step 2

18. Energy profile:

19. Examples

20. 2B. Modern Statement of Markovnikov’s Rule • In the ionic addition of an unsymmetrical reagent to an unsymmetrical double bond, the positive portion of the added reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate.

21. Examples

22. Examples

23. 2C. Regioselective Reactions • When a reaction that can potentially yield two or more constitutional isomers actually produces only one (or a predominance of one), the reaction is regioselective.

24. 2D. An Exception to Markovnikov’s Rule • Only with HBr in presence of peroxide. • Via a radical mechanism (see Chapter 10). • This anti-Markovnikov addition does not take place with HI, HCl, and HF, even when peroxides are present.

25. Stereochemistry of the IonicAddition to an Alkene attack from top racemate achiral trigonal planar carbocation attack from bottom

26. 4. Addition of Sulfuric Acid to Alkenes more stable 3o cation less stable 1o cation • Addition of H–OSO3H across a C=C bond.

27. 4A. Alcohols from Alkyl HydrogenSulfates • The overall result of the addition of sulfuric acid to an alkene followed by hydrolysis is the Markovnikov addition of H– and –OH.

28. Addition of Water to Alkenes:Acid-Catalyzed Hydration • Overall process: • Addition of H–OH across a C=C bond. • H+ is the electrophile. • Follow Markovnikov’s rule.

29. 5A. Mechanism

30. 5B. Rearrangements • Rearrangement can occur with certain carbocations.

31. Alcohols from Alkenes throughOxymercuration–Demercuration:Follows Markovnikov Addition. • Step 1: Oxymercuration • Step 2: Demercuration

32. 6A. Regioselectivity of Oxymercura-tion–Demercuration • Oxymercuration–demercuration is also highly regioselective and follows Markovnikov’s rule.

33. 6B. Rearrangements Rarely Occur inOxymercuration–Demercuration • Recall: acid-catalyzed hydration of some alkenes leads to rearrangement products.

34. See slide 30 also.

35. Rearrangements of the carbon skeleton rarely occur in oxymercuration–demercuration. no rearrangement

36. 6C. Mechanism of Oxymercuration There is no “free carbocation”.

37. Stereochemistry • Usually anti-addition.

38. Although attack by water on the bridged mercurinium ion leads to anti addition of the hydroxyl and mercury groups, the reaction that replaces mercury with hydrogen (step 2) is not stereocontrolled. This step scrambles the overall stereochemistry. • The net result of oxymercuration–demercuration is a mixture of syn and anti addition of –H and –OH to the alkene.

39. Solvomercuration-Demercuration Note: Step 1 with THF-HOH gives an alcohol. Step 1 with THF-ROH gives an ether.

40. Alcohols from Alkenes throughHydroboration–Oxidation:Anti-Markovnikov Syn Hydration • BH3 is called borane. • Addition of H–BH2 across a C=C bond.

41. BH3 exists as dimer B2H6 or complex with coordinative solvent.

42. An anti-markovnikov product results: syn addition Anti-Markovnikov addition of “H” & “OH”

43. Compare with oxymercuration-demercuration: anti addition Markovnikov addition of “H” & “OH”

44. Hydroboration: Synthesis ofAlkylboranes

45. 8A. Mechanism of Hydroboration

46. Other examples

47. 8B. Stereochemistry of Hydroboration • Syn addition

48. Oxidation and Hydrolysis ofAlkylboranes B always ends up on the least hindered carbon

49. Oxidation

50. Via